1. Field of the Invention
The invention relates to a process for producing a composite part, particularly for producing a part of a complex shape and a structural part.
2. Discussion of the Background
A first technique for producing composite parts consists in using a compression molding process. These processes for producing composite parts by compression molding consist of forcing a paste composed of short reinforcing fibers impregnated with resin to fill a compression mold under high pressures. These processes are widely used because of their suitability for the production of parts of complex shapes and because of their great productivity.
The preparation of the paste intended to be molded is a key step in the compression molding production process.
A first paste preparation process consists of performing a thread-by-thread coating of a reinforcing roving with a resin stabilized in an intermediate state. This resin is either in solution in a solvent ("solvent" impregnation) or in the molten state ("hot melt" impregnation). The impregnated roving is then cut into short fibers and stored before being compression molded. However, this process is not applicable to mass production and can create problems of industrial hygiene.
In a second process, the reinforcement and the resin are mixed in a mixer, yielding a paste ready to be molded ("bulk molding compound" or "BMC" technique). However, the impregnation quality is difficult to control, and the reinforcing fibers are often broken during the mixing, which gives rise to mechanical properties that are not always constant. Finally, the fiber volume rates ("FVR") attained are low (about 30%), which does not make it possible to produce structural parts.
A third process consists of depositing cut fibers continuously on a support film, adding resin to them, calendaring the resin and cut fibers between two plastic films and storing it to attain the desired viscosity. The sheets obtained are then cut off and stacked before being compression molded ("sheet molding compound" or "SMC" technique). With this process, again, the impregnation quality is not optimal, and the stacked sheets do not have the theoretical density of the finished product, which requires the removal of a considerable amount of air during molding. Furthermore, the arrangement in sheets leads to problems of rheology during the compression molding.
Another technique for producing composite parts consists of placing reinforcing fibers in a mold (production of a "preform" of the part consisting only of reinforcing fibers with, if necessary, a binder to assure its hold), then injecting into this mold, which is under vacuum and at low pressure, a liquid resin to impregnate the reinforcing fibers. The polymerization of the resin then starts in the mold with optional post-curing after removal of the part from the mold, obtain optimal mechanical properties ("resin transfer molding" or "RTM" process). A mold which makes it possible to inject resin under vacuum and at low pressure is called a "transfer mold".
This process makes it possible to obtain composite parts having excellent mechanical properties due, in particular, to the good impregnation of the reinforcing fiber with the liquid resin. However, this process is not well suited to the production of composite parts of complex shapes because of the difficulty in preplacing the reinforcing fibers quickly and satisfactorily in the transfer mold.
With regard to the prior art, the invention aims to improve the processes for making composite parts of complex shapes, particularly in the case of structural parts.